FIELD OF THE INVENTION
The invention relates to a method for operating a gas and steam turbine plant, in which heat contained in pressure-relieved flue gas of the gas turbine is utilized to produce steam for the steam turbine. It also relates to a gas and steam turbine plant that operates according to the method.
In a gas and steam turbine plant, the heat contained in the pressure-relieved flue gas of the gas turbine is utilized to produce steam for the steam turbine. The heat transfer takes place in a steam generator, which is located downstream of the gas turbine and in which heating surfaces, in the form of tubes or banks of tubes connected into the water-steam loop of the steam turbine, are disposed. The water-steam loop typically includes two pressure stages, and the heating surfaces of each pressure stage are subdivided into a preheater and an evaporator as well as a superheater. One such gas and steam turbine plant is known, for instance, from European Patent No. 0 148 973 B1.
When the pressure-relieved flue gas entering the steam generator is at a high temperature, and when there is a large total quantity of water available in the water-steam loop, especially low temperatures of the flue gas leaving the steam generator are achieved. That means that in full-load operation, the efficiency of the system is especially high. Typically, the parts of the system acted upon by the pressure-relieved flue gas, that is the steam generator or waste heat boiler and its built-in fixtures, are therefore constructed for full-load or rated-load operation.
However, during operation of such a system, the quantity of heat introduced into the steam generator can vary under various operating conditions. Critical operating states often arise as a result of the variable dynamic performance of the plant components, or in other words because of the comparatively long delay or reaction times of the steam turbine system as compared with the gas turbine system. For instance, in startup operation or upon load changes, and particularly in so-called fast starts or quick load changes, major temperature gradients and/or changes in the temperature over time, as well as sudden changes in the mass flow of pressure-relieved flue gas arise. Those changes, if they exceed certain maximum values over the course of time, can cause damage or can shorten the life of the system parts. Therefore, that kind of operation can endanger the steam generator.
The gas turbine system is therefore typically throttled in the startup and load change modes. That is generally done by controlling the quantity of fuel delivered to the gas turbine combustion chamber per unit of time as a function of the particular allowable changes in the pressure-relieved flue gas. In an extreme case, the pressure-relieved flue gas is diverted upstream of the steam generator through a bypass chimney. In the case of a gas and steam turbine plant, that represents a limitation in system availability.
In a gas and steam turbine plant without a bypass chimney, a critical operating state arises particularly when the system is operating in the so-called simple cycle mode. In that operating state, only the gas turbine is used to generate electricity. The steam produced in the steam generator must then be carried directly into the water-steam loop without conversion of energy in the steam turbine. Further steam production in the steam generator is necessary to avoid drying out of the heat exchanger tubes from impermissibly high temperatures at the heating surfaces.
Although damage can be kept slight by using special, high-quality and therefore expensive materials for the applicable system parts, heretofore the options in a fast start or quick load change of the gas turbine were extremely limited, because of the high thermal strain on the system parts those situations involved. With that mode of operation, it is not possible to use catalysts to scrub the flue gas.